Light-emitting diodes (LEDs) belong to a class of the most efficient light sources among currently available light sources. In particular, white LEDs find a rapidly expanding share in the market as the next-generation light source to replace incandescent lamps, fluorescent lamps, cold cathode fluorescent lamps (CCFL) for backlight, and halogen lamps. As one configuration for white LED, a white LED device (LED lighting device) constructed by combining a blue light-emitting diode (blue LED) with a phosphor capable of emitting light of longer wavelength, for example, yellow or green light upon blue light excitation is implemented on a commercial basis.
Among others, the current mainstream of the LED lighting device is a pseudo-white LED device comprising a blue LED and a yellow phosphor. Known examples of the yellow phosphor include Y3Al5O12:Ce, (Y, Gd), (Al, Ga)5O12:Ce, (Y,Gd)3Al5O12:Ce, Tb3Al5O12:Ce, CaGa2S4:Eu, (Sr, Ca, Ba)2SiO4:Eu, and Ca-α-SiAlON:Eu.
In these pseudo-white LED devices, for example, a phosphor member (wavelength conversion member) composed of silicone resin or glass having yellow phosphor particles dispersed therein is disposed forward of the blue LED whereby the phosphor emits yellow fluorescence of center wavelength around 570 nm (wavelength 510 to 600 nm) in response to incident blue light of wavelength around 450 nm. The yellow light is combined with the light of the blue LED transmitted by the wavelength conversion member to produce pseudo-white light.
However, the pseudo-white LED device using a typical yellow phosphor, Y3Al5O12:Ce emits light containing little of a green wavelength component of wavelength near 500 nm or a red wavelength component of wavelength 600 nm or longer. As a result, the light output of the pseudo-white LED device has a color rendering inferior to natural light, incandescent lamps and three-wavelength fluorescent lamps.
Thus, from the past, attempts are made to improve the color rendering of white LED devices. Among others, one attempt that intends to improve red light emission characteristics so as to endow the light output with a warm comfortable sense is to admix a red phosphor with a yellow or green phosphor in the wavelength conversion member of the pseudo-white LED device. Typical of the phosphors used are well-known nitride phosphors such as Sr—CaAlSiN3:Ee2+.
Among the white LED structures, the mainstream is a system in which a phosphor in admixture with resin or glass is disposed on or close to a blue LED, and the phosphor layer substantially integrated with the blue LED causes wavelength conversion to part or all of blue light for producing white light, which is referred to as white LED element.
Besides the method of mixing a yellow or green phosphor with a red phosphor to form a wavelength conversion member as mentioned above and the method of arranging a red emitting LED and a laser diode (LD) in parallel, means for adding a red component to light emission has not been proposed.